In general, doors are classified into a hinged type and a sliding type depending on its open/close mechanism. In the case of the hinged type, a hinge structure is installed between a wall and a door to allow the door to be opened or closed as the door is pushed or pulled. In the case of the sliding type, a rail is installed above or below the door, and the door is opened or closed by sliding along the rail.
In the sliding type, an integrated module of a roller assembly and a damper also slides along the rail provided above the door. In this case, if a significantly long rail is deformed, or the rail is irregularly deformed by an external force, it is difficult to appropriately drive the roller assembly and the damper designed to internally slide along the rail due to the deformed rail.
An extension rail is provided in straight alignment with the rail in order to allow a user to perform maintenance or inspection by removing the roller assembly and the damper from the rail. However, if the rail is deformed as described above, it is difficult to separate the extension rail and then remove the roller assembly and the damper from the rail through the removed space.